Field of the Disclosure
The field of the disclosure relates to radio-frequency (RF) identification (RFID) tags, also referred to as transponders, and particularly to powering RFID tags from RF field energy.
Technical Background
It is well known to employ radio frequency (RF) identification (RFID) transponders to identify articles of manufacture. RFID transponders are often referred to as “RFID tags.” For example, a RFID system could be provided that includes one or more RFID tags. The RFID tags may include RF circuitry in the form of an integrated circuit (IC) chip that is communicatively coupled to an antenna. The IC chip may also be coupled to memory. An identification number or other characteristic is stored in the IC chip or memory coupled to the IC chip. The identification number can be provided to another system, such as the RFID reader, to provide identification information for a variety of purposes.
If the RFID tag is an “active” tag having a transmitter, the RFID tag can transmit the identification information to a RFID reader using power stored in the RFID tag. Thus, an active RFID tag contains its own power source, which is typically a battery, for powering an RF transmitter. In contrast, if the RFID tag is a “passive” tag, the RFID tag does not contain its own power source. Power to operate a passive RFID tag is received through energy contained in a wireless RF signal received by the RFID tag antenna. The wireless RF signal is transmitted by a transmitter in the RFID reader. A passive RFID tag harvests energy from the electro-magnetic field of the wireless RF signal to provide power to the IC for a passive RFID tag operation and for communications with the RFID reader. A passive RFID tag can respond to receipt of the wireless RF signal from an RFID reader, including by providing identification information stored in the passive RFID tag, such as via backscatter modulation communications, as an example. In either case of a passive or active RFID tag, the RFID reader may store information received from the RFID tag in a database and/or report the information to other systems outside the RFID system.
It may be desirable to provide a RFID system that can detect events for a plurality of RFID tags. It may be desired to detect these RFID tag events as they occur. In this example, the RFID tags may be equipped with event detection capability. For example, events may include connection of the RFID tag to another electrical component, connection of a connector housing the RFID tag to another connection, or activating a switch associated with the RFID tag, as non-limiting examples. Events may also include detecting environmental conditions, including but not limited to temperature, pressure, humidity, or light exposures, as non-limiting examples. Some conditions, including environmental conditions, may require the RFID tags to be equipped with a condition event sensor capable of detecting the condition. A RFID reader provided in the RFID system may communicate with the entire RFID tag population to determine which RFID tags detected an event and the type of event that occurred.
An important limitation of passive RFID tag technology is that when insufficient RF power is available from a reader, the RFID tag will be inactive. This is illustrated by example in FIG. 1. FIG. 1 is a graph 10 illustrating RF power 12 received from a RFID reader by a RFID tag antenna of a RFID tag connection in decibels per meter (dBm) as a function of time (in seconds). A nominal threshold power is required to be received by the RFID tag antenna to turn on the RFID tag for operation. This is shown by power level line 14 in FIG. 1 and is assumed to be −16 dBm. In FIG. 1, an RFID tag is experiencing received power fluctuations, because the RFID reader switches its signal among four different RFID reader antennas, with equal time devoted to each RFID reader antenna. The RFID reader changes RFID reader antennas about once per second. The four different levels of RF power received by the RFID tag antenna are illustrated in FIG. 1 by power level 16A (about −5 dBm), power level 16B (about −10 dBm), power level 16C (about −18 dBm) and power level 16D (about −20 dBm), respectively, for the four RFID reader antennas. These different power levels occur because the RFID tag is located a different distance from each of the four RFID reader antennas.
However, as shown in FIG. 1, the RF power 12 received by the RFID tag antenna from the RFID reader may not always be at or above the nominal threshold power level 14. As illustrated in FIG. 1, the received RF power 12 is below the nominal threshold power level 14 about one half of the time. This is also referred to as negative power margin for the RFID tag. As also illustrated in FIG. 1, the RF power 12 received by the RFID tag antenna from the RFID reader is above the nominal threshold power level 14 about one half of the time. This is also referred to as positive power margin for the RFID tag. Two of the received RF power levels 16C, 16D are below the nominal threshold power level 14 for operation of the RFID tag and experience negative power margin. Two of the received RF power levels 16A, 16B are above the nominal threshold power level 14 for operation of the RFID tag and experience positive power margin. The RFID tag will turn on and off as RFID tag experiences positive and negative power margin. This negative power margin experienced at times by a RFID tag can be a problem in various situations.
As one non-limiting example, negative power margin in a RFID tag can occur when the RFID tag is shadowed by objects or other RFID tags. Negative power margin can also occur when metal is in close proximity to the RFID tag causing an impedance mismatch between the RFID tag and its RFID tag antenna. Negative power margin can also occur when reflections of the RF field of wireless RF signals cause interference or a null in the region of the RFID tag. Negative power margin can also occur if a RFID reader transmits wireless RF signals at a frequency for which the RFID tag antenna of the RFID tag is unresponsive. Negative power margin can also occur due to a RFID reader switching between different RFID reader antennas. Negative power margin can also occur when a RFID reader switches to a RFID reader antenna that is located too far (i.e. out of range) from the RFID tag to provide sufficient RF power to the RFID tag. All of these exemplary conditions, and others, can lead to periods of time during which the RFID tag does not harvest enough RF power from the wireless RF signals to power the RFID tag, thus rendering the RFID tag inoperable.